Channels and Transporters Flashcards
What did they discover from single channel recordings?
Microscopic (single channel) currents sum to the macroscopuc (whole-cell) currents.
Single Channel IV Plots
Changing the # of ions changes conductance and slope of IV plot.
Conductance is a combined measure of what?
Conductance is a combined measure of permeability and number of ions.
increase ion concentration = increase conductance = increase slope
Permeability: # of open channels (membrane property), doesn’t change in a single channel.
Conductance: ability of ions to pass through membrane; changes as # of ions changes.
What happens when you change concentration gradient
Changing the concentration gradient changes Erev and the x-intercept of IV plot
The single channel currents confirmed what?
The single channel currents confirm h&h model of action potentials
Na+ = negative inward current
little dely in opening and inactivates (transcient)
K+ = positive outward current
delay in opening, non-inactivation
Equation to go from micro to macro
Macroscopic IV plots
What are the macroscopic properties of Na+/K+ channels? Where do they result from?
Non-ohmic IV plots
Some channels are unable to pass current in both directions
* Inward rectifiers make the cell easier to excite.
* Outward rectifiers make the cell easier to hyperpolarize (easier to let current flow out).
Ion channel structure
Na+ channels:
* 10 human Na channel genes
* Na & Ca channels have 4 repetitive motifs and 6 membrane-spanning regions
* can incorporate beta-subunit adaptors
Ca+ channels:
very similar to Na+ channels
How to deduce structure of ion channels?
- Genetic sequence: motifs and mutagenesis
- Crystal structure: X-ray crystallography to deduce tertiary structure.
- Imaging: electron microscopy analysis of single channels
Ion transporters come in 2 forms:
- ATPases
- ion exchanges
Na+/K+ ATPase
because it removes 3 cations for every 2 it imports, the ATPase generates an outward current.
This can be detected as a small, slow outward current (hyperpolarizing).
How do you construct an IV plot?
IV plots are constructed using step depolarizations (voltage-clamp) while measuring the current.
What happens if you increase external [Na]?
What happens if you decrease external [Na]?
What happens if you increase external [K]
- increase external [Na] = shift to right ENa = more positive
- decrease external [Na] = shift to left, Ena decreases
- increase external [k] = shifts to right, Ek increases
